This invention relates to data transmission systems in which intelligence is conveyed in pulse code modulation (PCM) form, and especially to the use of such systems in automatic telecommunication exchange systems.
Balanced representation of a PCM bit stream is desirable to minimize the average DC (direct current) level on the line, but certain of the codes providing this facility, e.g. Alternate Mark Inversion (AMI), require line equalization. The economic penalty of this may be small for junctions, but it could be significant for subscriber circuits. However, advances in the techniques of automatic and adaptive equalization make the inclusion of such functions less objectionable. Where distances are relatively short the amount of equalization may be small or even reduced to reshaping the waveform. Hence, for PABX extensions and even for subscriber main exchange lines, such codes may be useful.